A (meth)acrylic acid-containing gas obtained through vapor-phase catalytic oxidation of propane, propylene, isobutene, or (meth)acrolein is collected in water or in a high boiling point solvent, to thereby form a (meth)acrylic acid solution. The (meth)acrylic acid solution is then subjected to a subsequent purification step involving extraction, diffusion, distillation, and crystallization, to thereby purify (meth)acrylic acid.
When a vaporization of (meth)acrylic acid as in distillation is involved in the purification step, an operation is conducted under reduced pressure. This is because reduced pressure decreases an operating temperature to suppress polymerization of (meth)acrylic acid. In addition to an operation of reducing pressure, air or a mixed gas of air and nitrogen (hereinafter, may also be referred to as “mixed gas”) may be supplied into a system for suppressing polymerization.
The supplied air or the mixed gas is guided to a vacuum device used for reducing the pressure along with a vapor of (meth)acrylic acid. When a steam ejector is used as the vacuum device, (meth)acrylic acid is trapped in condensed water of a steam discharged from the ejector. When a vacuum pump is used, (meth)acrylic acid is trapped in a seal water thereof.
An example of a technique of recycling the thus-obtained aqueous solution of (meth)acrylic acid includes a method for circulating the aqueous solution to a collecting step of collecting in the above-mentioned water or the high boiling point solvent crude (meth)acrylic acid formed in production of (meth)acrylic acid or to a purification step thereafter (see JP 2000-351749 A, for example). However, in the method, a large volume of water is circulated along with (meth)acrylic acid, and thus, at least energy required for purification of (meth)acrylic acid from the aqueous solution is further increased.
Air or the mixed gas is supplied to (meth)acrylic acid stored in a tank for preventing polymerization. The supplied gas is discharged from the tank as a vent gas along with a vapor of (meth)acrylic acid. (Meth)acrylic acid has a strong odor, and thus is usually guided to a deodorization column and trapped in water or an aqueous solution of caustic soda (see “Safety guidelines for handling acrylic acid and acrylates”, 5th edition, Japan Acrylate Industry Association, 1997, for example). The thus-obtained aqueous solution of (meth)acrylic acid has been heretofore treated as waste water. Thus, treatment of the aqueous solution of (meth)acrylic acid has involved a loss of (meth)acrylic acid and an increase in load of waste water treatment.
Meanwhile, JP 2000-351749 A discloses a method for directly introducing a vent gas containing (meth)acrylic acid into the collecting step. However, (meth)acrylic acid is usually produced through a continuous process, and a volume of a vent gas generated from the tank varies depending on receiving or discharge of (meth)acrylic acid. The method may become a variable factor of an operation in a continuous process, and thus is not preferable.
Further, a vent gas from the tank cannot be recycled while a production process of (meth)acrylic acid is stopped, and the vent gas from the tank must be treated separately.